Electrolytic preparation of organolead compounds



United States Patent 3,298,939 ELECTROLYTIC PREPARATION OF ORGANO- LEAD COMPOUNDS Jack Linsk, Highland, Ind., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana No Drawing. Filed Nov. 4, 1960, Scr. N0. 67,17 13 Claims. '(Cl. 204-59) This invention relates to organometallic compounds, and more particularly it relates to improvements in an electrolytic process for preparing tetra-alkyl lead compounds such as tetraethyl lead or tetramethyl lead.

Tetraethyl lead and tetramethyl lead are important organometallic compounds of commerce. It has hereto fore been discovered that tetra-alkyl lead compounds such as these may be prepared by electrolyzing an alkyl Grignard reagent, e.g., an ethyl magnesium chloride, using a lead anode. By this procedure, alkyl groups in the Grignard reagent are transferred to a lead anode, forming tetra-alkyl lead and giving magnesium chloride as a byproduct. This electrolytic process is superior to purely chemical processes by reason of the low capital investment and low material costs.

In carrying out the electrolytic process referred to above, the electrolysis may be effected with an ether solvent. In such a process separate phases are formed during the electrolysis, i.e., a first liquid phase containing unconverted Grignard reagent, a minor portion of the tetra-alkyl lead compound, magnesium halide and ether, and a second liquid phase containing substantially only tetra-alkyl lead compound and ether. When employing an ethyl magnesium chloride Grignard, the amount of magnesium chloride etherate in solution increases during the course of the electrolysis, and there is a tendency for magnesium chloride to be precipitated from the solution. One result is that the viscosity of the electrolyte increases and the electrolyte conductivity decreases which, in turn, increases the power requirements for the process. Further, the ether is attacked by the Grignard reagent at higher temperatures, which results in gassing and useless loss of raw materials. The ethers employed in the process frequently are expensive materials and it is important that ether decomposition be minimized. r

The main object of this invention is an improved process for the conversion of alkyl Grignard reagents to tetra-alkyl lead compounds by electrolysis usinga lead anode, which process includes the use of a particular 3,298,939 Patented Jan. 17, 1967 ice operating efiiciency and the economics of the electrolytic process. Benefits obtained from the use of such a mixed solvent include the improvement of both the electrolyte viscosity and conductivity, a lessened tendency for ether decomposition and the replacement of a portion of the expensive ether with a less expensive solvent component.

The dialkyl ethers of ethylene glycols employed herein are the dialkyl ethers of ethylene glycols having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion. Thus, the ethers have a formula R-O(C H -O) R, where the Rs are the same'or different alkyl groups, each having at least two and suitably up to 12 carbon atoms per group, and where n is either 1 or 2. Ethers outside the foregoing definition tend to form a magnesium chloride etherate which precipitates from the electrolyte, thereby fouling equipment, or, they may have undesirable physical properties, i.e., boiling point or they may be water miseach molecule of magnesium chloride formed in the electrolysis. For instance, in the process:

wherein the solvent is dibutyl Carbitol and benzene, the weight percent dibutyl Carbit-ol to be used for a 2 N Grignard solution would be approximately 44%, since the molecular weight of the ether is 218. However, it is desirable to keep to a minimum the amount of ether employed in the solvent mixture, and smaller amounts, as

K low as about 30 weight may be employed, depending upon the concentration of the Grignard solution.

The alkyl Grignard reagents are chosen to produce the desired tetra-alkyl lead compound. For example, an

ethyl Grignard will afford tetraethyl lead, while a methyl Grignard will produce tetramethyl lead. The alkyl lead solvent to provide improved operating efficiency and lower operating material costs.

In accordance with the present invention, the electrolysis of an' electrolyte comprising an alkyl Grignard reagent,exces"s' alkyl halide, and an ether, with a lead anode, is effected with a solvent mixture comprising a dialkyl ether of an ethylene glycol having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion and aromatic hydrocarbon. In a preferred embodiment of the present invention, tetrahydrofuran is also employed in conjunction with the above-mentioned solvent mixture to further increase the conductivity of the electrolyte.

It has been discovered that in the electrolytic process described above aromatic hydrocarbons can be employed in a mixed solvent comprising-a dialkyl'ether of an ethylene glycol having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion to homogenize the electrolyzed solution throughout the electroylsis operation and to improve both the compounds having about 1 to about 4 carbon atoms per alkyl group are most effectively produced by the electrolytic process. By employing mixed Grignard reagents, such as a mixture of ethyl Grignard and a methyl Grignard, a mixed tetra-alkyl compound may be prepared; thus, dimet-hyl diethyl lead maybe obtained. The halide portion of the alkyl Grignard reagent may be the chloride, bromide, or, less desirably, the iodide.

Grignard reagents are well known and no discussion of their preparation is necesary to describe the' present I the theoretical.

invention.

It has heretofore been discovered that the presence of excess alkyl halide in the electrolyte is of major advantage. Alkyl halide concentrations of, say, 1-50 weight percent of total electrolyte, react with magnesium metal which otherwise plate out on the cathode and short the electrodes; it assists in maintaining a high level of elec trolyte conductivity; it reduces the side reactions; and it appears to provide a current eificiency well in excess of With respect to the last mentioned point, this perhaps is due to the formation of a dialkyl lead compound electrolytically which may then react with Grignard reagent chemically to form tetra-alkyl lead, requiring only two Faradays per mole. Ordinarily, the alkyl halide will correspond to the alkyl Grignard reagent, i.e., ethyl chloride would normally be used with ethyl magnesium chloride. However, these alkyl groups need not be the same.

Heretofore, it has been found that tetrahydrofuran, when incorporated in the solvent mixture, has remarkable ability to increase electrolyte conductivity, frequently by a factor of 3 to 5 fold. The tetrahydrofuran may be employed within a broad range, i.e., from about 2 to 60 percent by weight, optimally, from about 5 to 50 percent by weight, and preferably from about to 30 percent by weight. Excessive amounts of tetrahydrofuran tend to cause precipitation of magnesium chloride from solution as electrolysis proceeds. Tetrahydrofuran is available commercially and its preparation need not be described herein.

Surprisingly, it has been found that aromatic hydrocarbons, when employed in a mixed solvent with a polyglycol ether have a remarkable effect on improving the electrolytic cell operation, whereas other materials such as parafiinic hydrocarbons do not exhibit this effect. By employing aromatic hydrocarbon in the solvent mixture a homogenous, single phase solution is obtained, which results in a much improved cell operation. The decrease in electrolyte conductivity and the increase in electrolyte viscosity which otherwise occurs during the course of a run are attributable, at least in part, to the formation of the magnesium chloride etherate complex. It is believed that the decrease in conductivity of the partially electrolyzed solution is related to the increase in viscosity, but, whether for this reason or others such as depletion of the Grignard reagent, or the coating of the electrodes with magnesium chloride etherate, the use of an aromatic hydrocarbon such as benzene in the solvent mixture decidedly reduces the tendency for electrolyte conductivity to decrease, while preventing an undesirable increase in viscosity.

The amount of aromatic hydrocarbon employed in the solvent mixture can vary over a fairly broad range, depending upon the molecular weight of the particular ether used. The preferred aromatic is benzene, however, other aromatic hydrocarbons can be used providing they are liquid at cell operating temperatures, and those which also possess suitable vapor pressures so that they may be readily separated from the alkyl lead or other components of the system are more desirable. These aromatics include toluene, xylene, ethylbenzene, mesitylene, cumene, cymene, etc. While small amounts of aromatic, i.e., about 23 weight percent, can be employed in the original solvent mixture, it has been observed that larger proportions of aromatic, as high as 50 weight percent or more, are desirable in the solvent mixture. With the smaller amounts of aromatic the electrolyzed solution tends to separate into two layers and the electrolyte conductivity drops as the electrolysis proceeds, depending upon the amount of alkyl halide present, the nature of the Grignard, the amount of tetrahydrofuran present, and other factors. The electrolysis can be initiated with lesser amounts of aromatic present, and subsequently, during the course of the electrolysis additional aromatic can be added to achieve the desired operational efiiciency and economy. The maximum proportion of aromatic is only limited by the conductivity of the electrolyte solution. For example, up to about 70 weight percent benzene has been employed in a solvent mixture with dibutyl Carbitol. Preferably, however, about 30-50 weight percent of aromatic is employed in the solvent mixture. It will be understood that either a single aromatic hydrocarbon or mixtures thereof may be employed in the solvent mixture.

Conditions in a typical electrolytic cell advantageously include a temperature within the range of about 20 C. to 100 C., preferably 20 C.-50 C., and optimally about 25 C.35 C. A current density within the range of about 0.2 to about 100 amperes per square foot is employed. Lower voltages of about 20-30 volts are preferred, however, up to 50 volts or higher may be employed. Cell pressures may range from atmospheric to superatmospheric, say 60 p.s.i.g., but preferably lower, i.e., less than about 30 p.s.i.g. Initial alkyl Grignard concentrations are advantageously within the range of about 1.53.5 Normal, and the electrolysis may be carried out in the presence of excess alkyl halide until the Gragnard concentration drops below about 0.2 N.

In a typical operation the product may be recovered in the following manner. After conclusion of the electrolysis the electrolyzed reaction mixture may be hydrolyzed with dilute hydrochloric acid, and the organic phase subjected to steam distillation for product recovery. Excess ethyl chloride (B.P. 12.2 C.) is distilled over initially, condensed and recovered. Benzene (or benzene and tetrahydrofuran) then distills at temperatures less than about C. The tetraethyl lead content of this overhead is less than about 1%. Then the tetraethyl lead distills off at about 100-104 C. and atmospheric pressure to obtain essentially pure tetraethyl lead. The dibutyl Carbitol or other ether solvent in the bottoms contains about 1 to 3% tetraethyl lead and may be dried and recycled to the Grignardpreparation step. The aromatic hydrocarbon, or aromatic and tetrahydrofuran mixture, is dried and then recycled to the electrolytic process.

To more fully describe the present invention the following illustrative examples are presented. It will be understood that these examples are given for the purpose of illustration only, and are not intended to limit the scope of the present invention.

EXAMPLE I A 2.72 molal methyl Grignard solution in a solvent containing about 15 weight percent tetrahydrofuran and dibutyl Carbitol is added to an electrolytic cell and allowed to circulate through the system. 41 g. of initial methyl chloride are added to the circulating solution while the system is being brought to an operating temperature of about 30 C. The electrolysis is started and additional methyl chloride added about every half hour (224 g.) during the run of 173 ampere-hours (40.25 hours total). Electrolysis is stopped when the Grignard concentration drops to 0.24 molal. During the run an average voltage of about 26 volts is maintained and the current drops from an initial 6.8 amps. to 1.9 amps. at the end of the run.

At the conclusion of the run the system pressure is reduced to zero and the gas collected and measured. The system is drained and the final solution measured and sampled. A summary of the results obtained from this run are as follows:

Grignard Conversion (OH) percent 80.9 TML Yield do 82.8 Yield based on Mg do 61.7 Initial THF Conc do 16. 8 Initial MeCl Conc. do 1.3 Gas Make do 4.42 Current Efliciency do 153 Current Density at 30 v., amp/ft. 5.2 Power Requiredkw. hr./#TML 3.12

At the end of the run before the products are removed from the cell, 1739 g. of benzene (ca. 40%) is added to the system. The current is turned on briefly and it is observed that the current increases from about 1.9 amperes to 8 amperes. The viscosity decreases decidedly and the electrolyzed solution is homogenized.

EXAMPLE II A 2.64 molal methyl Grignard solution is added in a solvent mixture containing about 12 weight percent tetrahydrofuran, about 28 weight percent benzene and dibutyl Carbitol to an electrolytic cell and the solution circulated through the cell. Then, about g. of initial methyl chloride are added while the system is brought to an operating temperature of about 30 C. I The electrolysis is started and additional methyl chloride is added about every half hour during the run of about'233ampere-hours (19 hours total). 'Durin'gthe run an average voltage of about 27.5 volts is maintainedQand the current dropped from an initial 21.8 amps. to 5.6 amps. at'the end of the run. Two phases are present at the conclusion of the run, i.e., about of an upper phase (0.02 molal) and about 90% of a'lowe r phase (0.16 molal). At the conclusion of the run the system pressure is reduced to zero, all the gas collected and measured and the system drained and sampled for analysis. A summary of the results of this run are as follows:

Grignard Conversion (OH) percent 86.3 TML Yield do 89.6 Yield based on Mg do 71.0 Initial THF Conc do 11.6 Initial Benzene Conc. ..do 28.6 Initial MeCl Conc. do 2.1 Gas Make do 2.52 Current Efficiency do 164 Current Density at 30 v., amp/ft. 14.6 Power Requiredkw. hr./#TML 3.04

There was apparently insufficient benzene present in the solvent mixture to maintain a completely homogenous solution near the end of this run. However, it is apparent that by incorporating benzene in the solvent mixture, a higher average current density is permitted, yield is increased and power required is decreased.

EXAMPLE III Electrolysis of the methyl Grignard solution is carried out in a manner similar to that described above, but in a solvent mixture containing about 45% benzene, about 10% tetrahydrofuran and about 45% hexylethyl Carbitol. The initial Grignard concentration is 1.30 millimols per gram and the initial methyl chloride concentration is about 2.0 weight percent. The electrolysis is started,

additional methyl chloride is added during the run and a current efficiency of 161% is maintained during the run. The electrolysis is conducted at an average temperature of 40 C. for a total of 219 ampere-hours (11.5 hours total) until a final Grignard concentration of 0.05 millimol per gram is reached. During the run the current dropped from an initial 21.5 amps. to 16.8 amps. at the end of the run, for an average of 19.1 amps. at an average voltage of 22.2 volts. The current density is 26.6 amps. per square foot at 30 volts. The final solution is one phase. At the conclusion of the run the system pressure is reduced to zero, all the gas bled being collected and measured and the system drained and sampled for analysis. The results of the run are summarized as follows:

Grignard Conversion (OH) percent 94.8 TML Yield do 99.1 Yield based on Mg do 94.0 Initial THF Conc. do 9.8 Initial MeCl Conc. do 2.0 Initial Benzene Conc. do 45.6 Gas Make ..do 0.68 Current Efiiciency do 161 Current Density at 30 v., amps/ft. 266 Power Required-kw.hr./#TML 2.50

From the above example it is readily seen that by incorporating the solvent of the present invention in the electrolytic process at near optimum conditions a high current density may be maintained during the electrolysis producing a high yield of TML at a low gas make with a reduced power requirement.

EXAMPLE IV A. 1.47 N ethyl Grignard solution in a solvent mixture containing about 17 Weight percent toluene, 11 Weight percent tetrahydrofuran, and 72 weight percent dibutyl Carbitol is electrolyzed as described above. The electrolysis is conducted for a total of about 20.8 amperehours at a cell temperature of 32 to 34 C. Initially the Grignard solution contained 3% ethyl chloride and additional ethyl chloride is added in increments during the electrolysis to correspond to about 200% of the amount of magnesium formed during the electrolysis. Initial current density is 65.6 amperes per square foot and near the end of the run, 34.3 amperesper square foot. Tetraethyl lead yield is 81.7%, based on Grignard consumed, and the gas make is 0.4%.

From the foregoing examples it is seen that the incorporation of an aromatic hydrocarbon in the solvent mixture of the above process permits an increased current density while resulting in increased product yield, and results in greater operational efliiciency and economy of the electrolytic process.

Having described my invention, what I claim is:

1. In a process for preparing a tetra-alkyl lead compound by electrolyzing an electrolyte comprising an alkyl Grignard reagent, excess alkyl halide, and an ether, with a lead anode, the improvement comprising effecting said electrolysis in the presence of a solvent mixture comprising a dialkyl ether of an ethylene glycol having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion, and aromatic hydrocarbon in an amount effective to reduce the tendency for the electrolyte conductivity to decrease during the course of said electrolysis.

2. The process of claim 1 wherein said solvent mixture comprises from about 30 to about 50 percent by weight of aromatic hydrocarbon.

3. The process of claim 1 wherein said dialkyl ether of an ethylene glycol is dibutyl ether of diethylene glycol.

4. The process of claim 1 wherein said dialkyl ether of an ethylene glycol is ethylhexyl ether of diethylene glycol.

5. The process of claim 1 wherein said aromatic hydrocarbon is benzene.

6. The process of claim 1 wherein said aromatic hydrocarbon is toluene.

7. In a process for preparing a tetra-alkyl lead compound by electrolyzing an electrolyte comprising an alkyl Grignard reagent, excess alkyl halide, and an ether, with a lead anode, the improvement comprising effecting said electrolysis in the presence of a solvent mixture comprising a dialkyl ether of an ethylene glycol having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion, aromatic hydrocarbon and tetrahydrofuran, said aromatic hydrocarbon being present in an amount effective to reduce the tendency for the electrolyte conductivity to decrease during the course of said electrolysis.

8. The process of claim 7 wherein said solvent mixture comprises from about 30 to about 50 weight percent of aromatic hydrocarbon and about 10 to about 30 weight percent of tetrahydrofuran.

9. The process of claim 7 wherein said dialkyl ether of an ethylene glycol is dibutyl ether of diethylene glycol.

10. The process of claim 7 wherein said dialkyl ether of an ethylene glycol is ethylhexyl ether of diethylene glycol.

11. The process of claim 7 wherein said aromatic hydrocarbon is benzene.

12. The process of claim 7 wherein said aromatic hydrocarbon is toluene.

13. In a process for preparing a tetraalkyl lead compound by electrolyzing an electrolyte comprising an alkyl Grignard reagent, excess alkyl halide, and an ether, with a lead anode, the improvement comprising effecting said electrolysis in the presence of a solvent mixture comprising a dialkyl ether of an ethylene glycol having at least two carbon atoms in each alkyl group and not more than two ethylene groups in the glycol portion, and an arcmatic hydrocarbon in an amount suflicient to increase the conductivity of the electrolyte.

References Cited by the Examiner UNITED OTHER REFERENCES Kondyrew, Berichte de D'eutsche Chemiche Gesellschaft, vol. 58 (1925), pages 459-463.

Pearson et a1., Transactions of the Eiectrochernical STATES PATENTS 5 Society, vol. 82 (1942), pp. 297-304.

Giraitis 20459 7 Giraifis et a1 kw 204 59 JOHN H. MACK, Przmary Exammer.

B ith jt 204 59 JOHN R. SPECK, JOSEPH REBOLD, Examiners.

Braithwaite 204-5'9 10 H. S. WILLIAMS, Assistant Examiner. 

1. IN A PROCESS FOR PREPARING A TETRA-ALKYL LEAD COMPOUND BY ELECTROLYZING AN ELECTROLYTE COMPRISING AN ALKYL GRIGNARD REAGENT, EXCESS ALKYL HALIDE, AND AN ETHER, WITH A LEAD ANODE, THE IMPROVEMENT COMPRISING EFFECTING SAID ELECTROLYSIS IN THE PRESENCE OF A SOLVENT MIXTURE COMPRISING A DIALKYL ETHER OF AN ETHYLENE GLYCOL HAVING AT LEAST TWO CARBON ATOMS IN EACH ALKYL GROUP AND NOT MORE THAN TWO ETHYLENE GROUPS IN THE GLYCOL PORTION, AND AROMATIC HYDROCARBON IN AN AMOUNT EFFECTIVE TO REDUCE THE TENDENCY FOR THE ELECTROLYTE CONDUCTIVITY TO DECREASE DURING THE COURSE OF SAID ELECTROLYSIS. 